Storm in a Teacup: The Physics of Everyday Life(14)

Gravity, like any other force, changes your speed—it accelerates you. This is a consequence of Newton’s famous second law,? which states that any net force acting on you will change your speed. When jumping off a diving board, you’re stationary to begin with, so you start to move slowly. The interesting thing about acceleration is that it’s measured in units of change of speed per second. At the start, you’re only getting going, so it takes a relatively long time (0.45 seconds) to fall through the first yard.But you go through the second yard much more quickly, so there’s less time to accelerate you on the way. After one yard, your speed is 4.6 yards per second, but after two yards it’s only 6.8 yards per second.

So you spend most of your time during a dive in the worst place, high above the water. In the first half of the time you spend in the air on a 5.5-yard dive, you only fall 1.33 yards. After that, things happen very quickly. Falling the full 5.5 yards takes one second, and by the end of that fall you’re traveling at 10.8 yards per second. You straighten out your body, reach for the water, and hope for a splash-free entry.

When competitions came around, the others in the group would eagerly take the opportunity to compete from the higher boards at whichever pool we were visiting. I did not. As far as I was concerned, more time in the air meant more time for things to go wrong. But this was never particularly logical, because you’re traveling so fast that falling through the extra distance doesn’t actually speed you up that much. It takes 1 second to fall 5.5 yards, but only 1.4 seconds to fall 11 yards. And you’re only going 40 percent faster, even though you’ve fallen twice as far. I knew that. But I was a diver for about four years and I have never once jumped off a board higher than 5.5 yards. I’m not scared of heights. I’m just scared of impacts. The longer gravity has had to accelerate me, the less pleasant it’s likely to be during the deceleration phase. Even dropping your phone is a reminder that letting gravity take over isn’t always a good idea. Extra distance to fall still provides the opportunity for extra speed . . . except when it doesn’t.

On Earth, there is a limit to what gravity can do to you. That’s because you are only accelerated by the overall force on you, called the resultant force. As you speed up, you have to push more air out of the way in a given period of time, and that air also pushes back on you, effectively reducing the pull of gravity because it’s pushing in the opposite direction. At some point, those two things balance, and you will travel at your terminal velocity, unable to get any faster. For leaves and balloons and parachutes, the force of the air pushing back is pretty big compared with the weedy gravitational pull, and so that force balance is reached at a relatively low speed. But for a human, terminal velocity close to the ground is around 120 mph. Sadly for any falling humans, air resistance is pretty negligible until you get to very high speeds. And it certainly doesn’t push back enough to reassure me about jumping off a 10-meter highboard, even now.


MY SCIENTIFIC RESEARCH is all about the physics of the ocean surface. I’m an experimentalist, and so part of my job is to go out on the ocean and measure what’s happening at this messy, beautiful boundary between the air and the sea. And that means spending weeks working on a research ship, a floating, functional, mobile scientific village. The problem with living on a ship is that you have to live with gravity basically having gone wrong. “Down” becomes an uncertain concept. Things may fall at the same speed and in the same direction as if you’d dropped them on land, but then again, they may not. If you spot a loose object just sitting on a table, you tend to find yourself watching it suspiciously because there is no guarantee that it’s going to stay put. Life at sea is full of elastic bungees, string, rope, sticky grip mats, locked drawers—anything that helps to keep life organized when there’s a capricious force pulling things in unpredictable directions, like a scientific poltergeist. My specific research topic is the bubbles produced by breaking waves in storms, and so I’ve spent months living at sea in some pretty nasty conditions. I actually quite like it—you adapt very quickly—but it’s a good lesson in how much we take gravity for granted. On one research ship in the Antarctic, the ship’s purser used to marshal the unreasonably enthusiastic among us for circuit training three times each week. We’d gather in the hold, an echoing iron space down in the guts of the ship, and obediently jump and lift and skip for an hour. It was probably the most effective circuit training I’ve ever done, because you never knew what force you were going to have to resist. The first three sit-ups might be ridiculously easy, because the ship was heaving downward, effectively reducing gravity. You’d just start feeling really good about yourself when the penalty arrived as the ship reached the bottom of the trough of the wave. At that point, gravity was effectively 50 percent stronger, and suddenly it felt as though your tummy muscles had to fight against strips of elastic pinning you to the floor. Four more sit-ups and gravity would vanish again. . . . Anything involving jumping was even worse because you were never quite sure where the floor was. And then afterward, in the shower, you’d spend your time chasing the water flow around the shower cubicle, as the rolling of the ship made it impossible to predict where it was going to fall.

Of course, there was nothing wrong with gravity itself. Everything on that ship was being pulled toward the center of the Earth with the same force. But when you feel the force of gravity, you’re resisting an acceleration. If your surroundings are accelerating all by themselves as the giant tin can you’re living in is tossed around by nature, your body can’t tell the difference between gravitational acceleration and any other acceleration that’s going on. So you get “effective gravity,” which is what you experience overall, without worrying about where it’s coming from. That’s why that odd feeling you get in an elevator only happens at the beginning and end of the ride–when the elevator is accelerating toward its top speed, or decelerating (a negative acceleration) toward a halt. Your body can’t tell the difference between the acceleration of the elevator and the acceleration due to gravity,? so you experience a reduced or increased “effective gravity.” For a fraction of a second, you can experience what it might be like to live on a planet with a different gravity field.